I'm looking to purchasing a house without a garage and saw that pre built garages were or garage kits were much more affordable than building a garage from ground up.
Anybody here have any experience with these garages? Are they just as good as any garage built by a contractor or are they going to collapse on me in 5-10 years?

Sorry, no experience with a kit. When I built my garage, I looked online and found a set of plans for a reasonable price. I paid for the slab, bought assembled trusses from a local company, and had Home Depot order me a garage door and header for it. I also had them deliver the materials. Everything else I did myself, except putting the trusses in place and lifting the header into place. This was about 7 or 8 years ago now, but the whole thing was very reasonable (I would guesstimate around <$10k for 24' by 24'). I already had a nail gun, chop saw, and all the other tools I needed. I got a little guff from the inspector about not using earthquake washers on the bolts holding the sill plates to the slab, but passed anyway. Oh, yeah, I did have a book on house building that was very helpful for figuring everything out. Planning was the hardest part. If the materials are good, I don't see any reason why a kit wouldn't provide a lifetime of use.

I would be leery of a "kit garage." Unless you have thoroughly researched the details of exactly what you will be getting for your money. Not speaking from having direct experience with kits, but the logical question is "how can the kit manufacturer be making a profit, unless he's skimping on materials?"

Maybe you could post a link to there site so could really pick on it. It could be a good product but what are you getting? You still need a foundation and it still has to be built, so I suspect you get a set of plans a truck load of material.

As stated it really depends on the company.
When I lived in NH there was companys that sold what's called panelized garages. a truck with a boom on it would show up with the walls all prebuilt, they just lifted them off the truck and set them in place, the siding, windows and doors were already installed except the over head door.
In one day to a day and 1/2 the garage would be done.

As far as kits watch out for all the "extras" there going to hit you with.
Some I've seen the standard kit did not include drip cap for the roof or tar paper, 3, tab shingles, T-111 siding, 2 X 3's every 24" for studs, no door knobs, cheap shed single pane windows.

If the walls are not pre built when there delived I see nothing to gain by buying a kit. Lowes, Home Depot both can design a garage or shed on the in house computer and spit out a materials list for you. A local lumber yard may even be able to help you with that. I know I can just leave off a set of plans and they will do the take off for me.

I built a 2 car 2 story garage with an attached shed for my home last year. I searched the Internet for a kit and/or designs that would work for my space but came up empty. I ended up purchasing an inexpensive CAD program and designing it myself. I double checked with a friend that's an engineer prior to giving the plans to our local building inspector for approval. Other than the slab, I did everything myself (with help of family and friends). I purchased dimensional lumber and cedar clapboard siding from a local lumber yard that I've done business with for years and purchased sheathing, decking and shingles from Lowe's. I put about 5k into the 2 overhead doors that I purchased from and that were installed by the Overhead Door Co. I built the man door for the garage and the double doors for the shed myself because the cost for the quality I wanted was too high. I spent a total of about 30k for everything except the driveway (and paint). I did not Sheetrock or insulate. The insurance company appraised it at 65k.

What I'm saying is he's right. You're not saving anything by having someone else put together a bill of materials for you. You're paying for that service. With help from your local building supplier, you can be your own GC and the local retailer is more likely to have plans already approved by your building inspector.